The present invention relates generally to a micro-electromechanical (MEMS) radio frequency (RF) switch, and more specifically, to a MEMS switch that operates with a low actuation voltage, has a very low insertion loss, and good isolation.
A radio-frequency (RF) switch is a device that controls the flow of an RF signal, or it may be a device that controls a component or device in an RF circuit or system in which an RF signal is conveyed. As is contemplated herein, an RF signal is one which encompasses low and high RF frequencies over the entire spectrum of the electromagnetic waves, from a few Hertz to microwave and millimeter-wave frequencies. A micro-electromechanical system (MEMS) is a device or system fabricated using semiconductor integrated circuit (IC) fabrication technology. A MEMS switch is such a device that controls the flow of an RF signal. MEMS devices are small in size, and feature significant advantages in that their small size translates into a high electrical performance, since stray capacitance and inductance are virtually eliminated in such an electrically small structure as measured in wavelengths. In addition, a MEMS switch may be produced at a low-cost due to the IC manufacturing process employed in its fabrication. MEMS switches are termed electrostatic MEMS switches if they are actuated or controlled using electrostatic force which turns such switches on and off. Electrostatic MEMS switches are advantageous due to low power-consumption because they can be actuated using electrostatic force induced by the application of a voltage with virtually no current. This advantage is of paramount importance for portable systems, which are operated by small batteries with very limited stored energy. Such portable systems might include hand-held cellular phones and laptop personal computers, for which power-consumption is recognized as a significant operating limitation. Even for systems that have a sufficient AC or DC power supply such as those operating in a building with AC power outlets or in a car with a large DC battery and a generator, low power-consumption is still a desirable feature because power dissipation creates heat which can be a problem in a circuit loaded with many IC""s. However, a major disadvantage exists in prior art MEMS switches, which require a large voltage to actuate the MEMS switch. Such a voltage is typically termed a xe2x80x9cpull-downxe2x80x9d voltage, and, in the prior art may be anywhere from 20 to 40 volts or more in magnitude and therefore not compatible with modem portable communications systems, which typically operate at 3 volts or less. To explain further, a typical MEMS switch uses electrostatic force to cause mechanical movement that results in electrically bridging a gap between two contacts such as in the bending of a cantilever. In general this gap is relatively large in order to achieve a large impedance during the xe2x80x9coffxe2x80x9d state of the MEMS switch. Consequently, the aforementioned large pull-down voltage of anywhere from 20 to 40 volts or more is usually required in these designs to electrically bridge the large gap. Also, a typical MEMS switch has a useful life of approximately 108 to 109 cycles. Thus, in addition to the above concerns, there is an interest in increasing the lifetime of such MEMS switches. Thus there is a need for an electrostatic MEMS switch that is actuated by a low pull-down or actuating voltage and has low power consumption with increased cycle life.
It is, therefore, an object of the present invention to provide a micro-electromechanical (MEMS) switch that operates with a low actuation voltage, and has a very low insertion loss and good isolation.
It is another object of the present invention to provide a fabrication process that is fully compatible with CMOS, BiCMOS, and SiGe processing, and can be monolithically integrated at the upper levels of chip wiring.
To achieve the above objects, there is provided a capacitive electrostatic MEMS RF switch comprised of a lower electrode that acts as both a transmission line and as an actuation electrode. Also, there is an array of fixed beams that is connected to ground above the lower electrode. The lower electrode transmits the RF signal when the upper beams are up, and when the upper beams are actuated and bent down, the transmission line is shunted to ground.